A variety of heat exchangers are used in a wide range of industrial fields, such as nuclear industry, aerospace industry, chemical industry, and general industry. In particular, heat exchangers required to be resistant to high temperature and corrosion use a ceramic block as the heat exchange section. As an example of such heat exchange, a heat exchanger will now be described hereunder which is used in a thermochemical IS process hydrogen production apparatus for producing a large amount of hydrogen and oxygen from water material using nuclear heat of about 950° C.
The IS process basically includes the following three sub systems.I2+SO2+2H2O=2HI+H2SO4: Bunsen reaction(exothermic)≦100° C.  (Chemical formula 1)
2HI=H2+I2: hydrogen iodide decomposition reaction (endothermic) 400° C.
H2SO4=H2O+SO2+½O2: sulfuric acid decomposition reaction (endothermic) 800° C.
The sulfuric acid decomposition and the hydrogen iodide decomposition of the three sub-systems are endothermic reactions and their systems are heated by heat exchange with, for example, high-temperature helium gas supplied from a high-temperature gas furnace in a heat exchanger.
Since sulfuric acid is corrosive to metals, the heat exchange section must be corrosion-resistant. In addition, the heat exchange section is used at high temperatures in the range of 400 to 800° C., and therefore, cannot use combinations of a polytetrafluoroethylene-based material and a metal generally used in sulfuric acid plants.
Accordingly, a sulfuric acid evaporator has been proposed in which the sulfuric acid decomposition reaction is conducted by using a ceramic block, from the viewpoint of enhancing the high-temperature corrosion resistance and the high-temperature strength (see, for example, Patent Document of Japanese Unexamined Patent Application Publication No. 2005-61785).
In this proposal, sulfuric acid of about 455° C. supplied to a lower plenum through a sulfuric acid supply pipe connected to the bottom of a pressure vessel containing high-temperature helium gas is delivered to a heat exchanging section and discharged from a sulfuric acid gas exhaust pipe through an upper plenum.
The heat exchange section includes two pairs of straight flow channels, a ceramic block having flow channels, a plurality of partition plates surrounding the ceramic block, and inner tube disposed so as to contain the partition plates.
A helium inlet nozzle extending from the pressure vessel to the inner tube is disposed at the upper side of the heat exchange section. Thus, high-temperature helium gas of about 689° C. supplied from a high-temperature heat exchange system flows into the top chamber of the heat exchange section. The high-temperature helium gas further flows through one of the flow channels of the ceramic block, meandering through chambers defined by the partition plates and the inner tube, and the heat is exchanged between the helium gas and the sulfuric acid flowing through the other flow channel. The helium gas flowing out to the pressure vessel from the bottom chamber of the heat exchange section is returned to the high-temperature heat exchange system through the helium outlet nozzle.
The ceramic block is generally produced by compacting a powder, cutting the compact to a predetermined size, and then sintering the compact. Ceramic is resistant to high temperature and suitable for use in the intended atmosphere, and is also corrosion resistant. In use for heat exchangers, it is an important factor that ceramic has a high thermal conductivity. In use for the heat exchanger of a thermochemical IS process hydrogen production apparatus using sulfuric acid, ceramic is required to have corrosion resistance to sulfuric acid and heat resistance.
In sulfuric acid plants for producing sulfuric acid or the like, which run at a low temperature of about 200° C., a corrosion-resistant material, such as polytetrafluoroethylene or glass, is coated with a metal.
In the heat exchanger for thermochemical IS process hydrogen production apparatus running at a temperature of 400° C. or more, however, polytetrafluoroethylene or the like cannot be used, and heat exchangers made of ceramic, such as silicon carbide or silicon nitride, has been proposed from the viewpoint of corrosion resistance to sulfuric acid and heat resistance. Unfortunately, ceramic is an elastic material, and has some disadvantages in use as a structural material in comparison with metals.
More specifically, first, ceramic is brittle and, accordingly, has low toughness. Consequently, a crevice extends from a point undergoing stress concentration or a tiny crack, and may result in destruction.
Second, ceramic is difficult to join. Metallurgical techniques for joining ceramic include: (1) brazing; (2) reaction sintering; and (3) pressureless sintering. In order to achieve reliable joining, a furnace is required, and the size of the materials to be joined to each other is limited by the capacity of the furnace. In addition, if a defect or uneven strength occurs at the joined portion, not only the strength is degraded, but also the above-mentioned crack extension may be caused by stress concentration.
Third, the linear expansion coefficient of ceramic is about 4E-6/° C. and is lower than metal structural materials, such as stainless steel (18E-6/° C.) and inconel (15E-6/° C.). Accordingly, a difference in thermal expansion occurs at high temperatures.
In particular, this problem is pronounced in the sulfuric acid evaporator. More specifically, the known heat exchange section disclosed in the above-cited Patent Document is produced by joining and integrating a plurality of ceramic blocks because of the limitation in forming one of the flow channels. The integrated size is limited by the capacity of the furnace for joining. An easy method, such as using adhesive, but not a furnace, does not provide a high-temperature strength as high as that of the base material. In addition, a discontinuous strength is exhibited at the junctions, and, consequently, the junctions may be destructed by stress concentration.
Furthermore, one flow channel and the other flow channel are orthogonal to each other. This makes it easy to separate helium gas flawing through one of the flow channels from sulfuric acid flowing through the other. However, the orthogonal flows running through the tubular flow channels come into apparent point contact with each other and their opposing area is small. Consequently, the quantity of exchanged heat is reduced. The differences in thermal expansion between the partition plates and the ceramic blocks and between the partition plates and the inner tube may provide a problem.
Since the inner tube is fixed to an end plate made of a metal, such as stainless steel, the inner tube and the partition plates are preferably made of a metal, such as stainless steel. However, if the partition plates and the inner tube are made of, for example, stainless steel, gaps will be formed between the peripheries of the ceramic blocks and the internal circumferences of the partition plates, and helium leaks from the gaps. Consequently, the quantity of exchanged heat is reduced. In order to prevent the leakage of helium gas resulting from the thermal expansion difference, some thermal expansion absorber may be required to be provided.